Most aerial tramway systems which are currently in use employ a haul rope which is an endless loop that is driven at one end by a drive bull wheel rotating in a horizontal plane. The opposite end of the haul rope loop also is supported on a horizontally oriented idler bull wheel. Intermediate sheaves, usually positioned on towers, support the haul rope and passenger carrier units as they move along the loop. Usually there is a station at each end proximate the bull wheels, and the passenger carrier units may be permanently affixed to the haul rope or demountably attached to the haul rope.
One problem which has been encountered in connection with aerial tramway systems which employ a single endless cable or haul rope is that the passenger carrier unit can swing laterally with respect to the haul rope as a result of wind loading and/or movement of the passengers in the unit. To lessen this tendency, a variety of hanger arms have been devised which will damp or resist lateral motion of the passenger carrier unit.
Another problem which has been encountered in connection with single rope aerial tramway systems has been that, as the steepness of the course increases, the hanger arm on which the cabin, gondola or chair is supported must also increase in length. As the steepness of the aerial tramway course increases, there is a tendency for the downhill end of the passenger carrier unit to engage the haul rope, since the passenger carrier unit must be mounted directly beneath the haul rope in single rope systems. This problem is increased by the necessity of mounting the passenger carrier unit to the haul rope in a manner which will permit some fore-and-aft swinging of the unit.
As the length of the cabin or gondola hanger arm increases to accommodate steepness in the tramway course, the potential for fore-and-aft swinging also increases, since the wind load on the cabin acts on a longer and longer moment arm. Similarly, the increased length in the hanger arm also increases the tendency for lateral swinging under dynamic loading because of the increased moment arm.
One approach which has been tried to reduce the lateral swing of the passenger carrier units in an aerial tramway system is to mount the cabins between a pair of haul ropes. Thus, lateral stability can be achieved if the passenger carrier unit grips a pair of side-by-side haul ropes which are simultaneously advanced in synchronism in the same direction. Such pairs of haul ropes have been mounted to both vertically and horizontally oriented, bull wheels. Typical of such a vertical bull wheel system is the aerial tramway assembly of French Patent Application published on May 9, 1980 as publication No. 2,448,464 and corresponding U.S. Pat. No. 4,509,430. In this aerial tramway four parallel, endless-loop haul ropes are mounted to four parallel, vertically oriented bull wheels. The passenger carrier unit is coupled to all four haul ropes by four grip assemblies, which provide the passenger carrier unit with extremely high stability against lateral swinging. The system, however, is relatively complex and inherently produces a new problem.
Any aerial tramway system in which the passenger carrier units are simultaneously coupled to more than one haul rope raises the problem of synchronizing the movement of the haul ropes. Even if the driving bull wheels are mounted to a common drive shaft, it is virtually certain that the ropes will not be advanced at precisely the same rate. For example, for a typical aerial tramway systems a bull wheel may have a diameter of about 10 feet. If two bull wheels are manufactured with an error in tolerances of only 0.02 inches on the diameter, these two bull wheels will produce a difference in the haul rope position of approximately 6.28 inches after only 100 revolutions. Similarly, differences in friction of the haul rope on the support sheaves under varying loads can produce errors in synchronism as larger or larger than bull wheel diameter differences.
It is very difficult to determine the speed of the haul rope except through inferential measurements. Thus, the tramway of U.S. Pat. No. 4,509,430, for example, to synchronizes the movement of parallel haul ropes based upon torque sensing, i.e., drive motor voltage and the use of an electric differential. The problem with this approach is that friction of the support sheaves on the rope may vary substantially and produce false torque sensing that result in erroneous and constant adjustments of the bull wheel drive torque.
Additionally, the system of U.S. Pat. No. 4,509,430 also employs relatively rigid coupling bars between adjacent haul ropes and the haul ropes are mounted close together to attempt to minimize differences or force synchronization. Mounting the haul ropes close together, however, reduces laeral cabin stability and requires a long hanger arm when used on steep slopes.
Another approach to the lateral stabilization of passenger carrier units on an aerial tramway is disclosed in French Patent Application Publication No. 2,525,981. In this patent two horizontally oriented bull wheels are used to provide the parallel haul ropes. This system is again faced with the problem of haul rope synchronization, which is made even more diffcult by the fact that the use of horizontal bull wheels results in cable paths which are not of the same length. Thus, the bull wheels cannot be operated conveniently off the same drive shaft.
A similar approach employing a single common bull wheel is set forth in French Patent No. 1,249,949. Thus, the system employs a bull wheel having two cable receiving grooves, but the problems of tolerances in manufacture still remain the same, namely, very small errors in the groove diameters can induce intolerable differences in the position of the two haul ropes.
It should be noted that even if both grooves, or both bull wheels, are manufactured to exactly the same tolerances, the rubber groove linings that are engaged by the haul rope cannot be assumed to deflect absolutely uniformly. Thus, even perfectly manufactured bull wheels will introduce differences in the position of the haul ropes which are driven as a result of uneven compression of the groove lining material.
Still a further aerial tramway system is disclosed in French Patent Application Publication No. 2,430,901 in which vertical bull wheels are used to drive parallel haul ropes. Permanently mounted between the haul ropes are conveyor buckets or tubs which simply pass around the vertical bull wheels and return in an inverted position. The problem of rope synchronization still exists, and since the units are permanently affixed to the haul ropes, the accumulative error can be quite substantial unless some provision is made to adjust the lack of synchronism between the haul ropes.
Accordingly, it is an object of the present invention to provide an aerial tramway system and method which affords the stability advantages of coupling of the passenger carrier units to side-by-side haul ropes while maintaining the synchronous operation of the haul ropes.
It is another object of the present invention to provide an aerial tramway system and method in which lateral swinging and fore-and-aft swinging of the carrier units are minimized.
It is another object of the present invention to provide an aerial tramway system and method in which multiple drive bull wheels can be operated from a common drive shaft while still maintaining synchronism in the displacement of the haul ropes driven by the bull wheels.
Still a further object of the present invention is to provide an aerial tramway system and method in which the passenger carrier units can be easily mounted to and demounted from the tramway haul ropes.
Still a further object of the present invention is to provide a bull wheel assembly for an aerial tramway which enables dynamic adjustment of the rate at which the haul rope is driven without changing the rate of rotation of the bull wheel to enable synchronization of the speed of advancement of the haul rope as compared to a parallel rope operating at the same speed.
Still a further object of the present invention is to provide an aerial tramway and adjustable bull wheel assembly which has enhanced safety, is easy to operate, is durable, and requires minimum maintenance.